Generally, rubbers have a high elongation and a low elastic modulus and have been practically made into composites with other materials such as plastics to make the best use of their properties. To produce such composites advantageously, a rubber and an adherend are integrally bonded by the vulcanization of the rubber, because such a method is easily and simply applicable even when the adhesive surface is geometrically complicated. To ensure an adhesion force, there are known various methods including the surface treatment of plastic adherends, the modification of components of adhesive composition, etc.
In conventional rubber bonding methods; using an adhesive composition, there have been extensively used solvent type adhesives comprising a rubber-containing adhesive composition and a solvent such as water and an organic solvent. The solvent is used to make the adhesive composition into liquid state, thereby ensuring a void-free coating of the adhesive composition on the adherend. To enhance the resistance to cohesive failure of the adhesive composition so as to prevent the adhesive joint from being broken by a stress such as dynamic strain, the solvent is frequently removed by evaporation, etc. in the adhesion step for hardening the adhesive layer. The adhesive composition requiring the removal of the solvent upon hardening the adhesive layer fails to utilize the solvent as an effective component after the adhesive-bonding, and often necessitates a solvent treatment for environmental protection. Therefore, the use of a solvent-free adhesive composition or an adhesive composition in which the solvent is incorporated into the adhesive layer in the adhesion step for hardening the adhesive composition is desired.
There has been conventionally known a bonding method using, as the adhesive composition of the above type, an adhesive composition containing a monomer that is polymerizable by the irradiation of radiation such as ultraviolet light, visible light and electron beam or by the application of external energy such as heat. In this method, the adhesive composition is in a liquid state when applied on the surface of adherend to thereby minimize adhesive defects due to voids, and the adhesive layer with a network cross-linked structure is formed by the polymerization of monomer in the adhesive-bonding treatment. Regarding the method of bonding rubber using the adhesive composition of this type, Japanese Patent Application Laid-Open No. 55-145768 (DE 2916909C) discloses that: a coating compound comprising at least one liquid hydrocarbon polymer having a double bond and (or), if required, at least one polyamine derivative is rapidly or completely made into network structure by the irradiation of ionizing radiation such as electron beam without external heating, thereby ensuring its good adhesion to a rubber substrate. It is further described in the document that: the coating compound may be applied to a completely vulcanized molded article and then made into network structure, or alternatively may be applied on a crude mixture not yet made into network structure and then made into network structure simultaneously with the crude mixture. The document proceeds that: in the latter treatment, if the vulcanization is conducted by the irradiation of electron beam, the use of sulfur as a vulcanizing agent and (or) a vulcanization accelerator for making the crude mixture into network structure can be omitted. The proposed method relates to making the rubber component and the adhesive composition into network structure to firmly bond onto rubber, and is characterized that the use of sulfur as a vulcanizing agent and (or) a vulcanization accelerator can be omitted in making the rubber and the adhesive composition into network structure. However, as a result of the studies on the adhesive composition and the bonding method of the present invention, the inventors have found that in the case of making the adhesive composition into network structure by the optional irradiation with irradiation such as ultraviolet light, visible light and electron beam, when the heating is made while bringing an unvulcanized rubber into press-contact with the adhesive composition, sulfur migrates from the rubber into the adhesive composition, thereby achieving a strong adhesion due to the sulfur-assisted cross-linking reaction.